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The Government response to the findings of the Overseer peer review report
Disclaimer
The information in this publication is, according to the Ministry for the Environment’s best
efforts, accurate at the time of publication. The Ministry will make every reasonable effort to
keep it current and accurate. However, users of this publication are advised that:
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requirements.
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those websites nor of organisations or people referred to.
This document may be cited as: Ministry for the Environment and Ministry for Primary
Industries. 2021. Government response to the findings of the Overseer peer review report.
Wellington: Ministry for the Environment and Ministry for Primary Industries.
Published in August 2021 by
Ministry for the Environment
Manatū Mō Te Taiao
PO Box 10362, Wellington 6143, New Zealand
Ministry for Primary Industries
Manatū Ahu Matua
PO Box 2526, Wellington 6140, New Zealand
ISBN: 978-1-99-003369-8
Publication number: ME 1582
© Crown copyright New Zealand 2021
This document is available on the Ministry for the Environment website: environment.govt.nz
and on the Ministry for Primary Industries website: www.mpi.govt.nz/news-and-
resources/publications
2 Government response to the findings of the Overseer peer review report
Contents
Message from the Ministers 4
Executive summary 5
1 Introduction 8
2 About Overseer 9
2.1 How Overseer is used 9
2.2 Positive features of Overseer 9
2.3 Policy context for Overseer use in regulation 10
3 Overseer whole-model review 11
3.1 How the Science Advisory Panel undertook its review 11
3.2 Review findings 12
3.3 Panel’s conclusions 13
4 The Government’s response 14
4.1 Ongoing implementation of planning and regulatory functions 14
4.2 Developing regional plans to meet the NPS-FM deadlines 16
4.3 Managing the risks of increasing nitrogen discharges 17
5 The way forward: the next 12 months 19
5.1 Ongoing support for regional councils 20
5.2 Understanding our current and future needs 20
5.3 Option (a): Nutrient risk assessment approach 20
5.4 Option (b): Amending Overseer to address Panel concerns 22
5.5 Option (c): Control of practices and inputs 22
5.6 Option (d): New management tools 23
Appendices 25
Appendix A: Science Advisory Panel 25
Appendix B: Existing use of Overseer 27
Appendix C: Examples of risk assessment approaches 31
Government response to the findings of the Overseer peer review report 3
Figure
Figure 1: Future work timeline 24
4 Government response to the findings of the Overseer peer review report
Message from the Ministers
There are three main causes of water quality degradation – excessive sediment, microbial
contaminants and/or nutrients. For many New Zealand’s rivers, the main nutrient challenge is
excessive nitrogen in its various forms.
Diffuse nutrient discharges from farmland and their risk to freshwater are managed in a variety
of ways, including through the use of the Overseer on-farm nutrient model. Overseer has
helped farmers and regional councils gather data on inputs like fertiliser and livestock
numbers, via an online, user-friendly interface. Using those data, it has sought to estimate the
extent of nutrient ‘loss’ from farm systems.
Industry and environmental groups have, for some time, debated the application of Overseer’s
nutrient loss modelling to regional councils’ planning and consenting functions.
In 2018 the Parliamentary Commissioner for the Environment recommended a review of
Overseer be undertaken. An independent Scientific Advisory Panel was subsequently
established by the Government to undertake this important work.
We thank the Scientific Advisory Panel for their careful review of the Overseer model. The
Panel has identified a range of shortcomings in the modelling approach. The review will help us
develop better tools for farmers and regulators to meet future Essential Freshwater
requirements.
The Panel’s findings require a response from Government. That is what this report provides. It
is apparent that, at least in its current state, Overseer does not provide reliable results across
the range of situations it is currently used for.
Overseer will be supported while the next generation of Overseer and additional tools are
developed and made available to help manage nutrient losses from farms. We will also assist
regional councils to continue with their freshwater plan-making, monitoring and compliance
work.
We will now progress the next, solutions-oriented, phase of this programme to address the
issues identified in the Panel’s report. The attached document describes that work and the role
that Overseer may play in the future.
Hon David Parker Hon Damien O’Connor
Minister for the Environment Minister of Agriculture
Government response to the findings of the Overseer peer review report 5
Executive summary
In late 2018 the Parliamentary Commissioner for the Environment (PCE) outlined steps that
should be taken for the Overseer model to be used confidently in regulation, including
recommending that a whole-model peer review of Overseer be undertaken by technical
experts independent of those who developed the model1. That review was subsequently
commissioned by the Ministry for Primary Industries (MPI) and the Ministry for the
Environment (MfE). The Science Advisory Panel (the Panel) that undertook the review has now
reported its findings. The Government thanks the Panel members for their hard work over the
past year.
Overseer is a tool commonly used to inform our understanding of nutrient loss across
agricultural landscapes and the effect of farm practices on those nutrient losses, at both
farm and catchment scales. Although Overseer was conceived as a decision-support tool for
agricultural producers (principally to inform fertiliser application decisions), it has evolved over
the years to become a tool used by regulators to enable limits to be set for diffuse nitrogen
discharges – compliance with which is required to be assessed using Overseer modelling.
Overseer does not model diffuse discharges of sediment or pathogens. Overseer’s role in
existing regulation is variable across the country but, where used, generally forms part of a
wider suite of controls, including direct controls on practices and inputs and a requirement for
farm planning, enabling farm-specific risks across all four main diffuse contaminants to be
identified and addressed.
In 2020, the Government released the Essential Freshwater reforms. Important components
of the reform package included the National Policy Statement for Freshwater Management
(NPS-FM) 2020 and the Resource Management (National Environmental Standards for
Freshwater) Regulations 2020 (NES-F). Neither of those instruments require the use of, or refer
to, Overseer modelling. Nevertheless, it has been expected that Overseer would be used for
implementation of some elements of those instruments.
In that context, the review is important. If Overseer is to be used to manage nutrients, we
need to have a high level of confidence in its modelling estimates.
The Panel concluded that, in its current form, it would not have confidence that Overseer’s
estimates of nitrogen lost from farms are suitable for use as a stand-alone measure of total
losses. The Panel concluded that, in its current form, it would not have confidence in
Overseer’s estimates of nitrogen lost from farms2. The Panel identified a general concern with
the model structure as well as some specific areas of concern. Importantly though, the Panel
did not assess the model for ease of use, its data files, nor its use as a social tool to encourage
farmers to adopt farm management strategies that would successfully reduce a catchment’s
nutrient load.
The specific areas of concern include that Overseer:
• does not balance mass (that is, some nitrogen is not accounted for or is unexplained)
1 Parliamentary Commissioner for the Environment, 2018. Overseer and regulatory oversight: Models,
uncertainty, and cleaning up our waterways.
2 In accordance with its terms of reference, the Panel did not consider Overseer’s modelling of greenhouse
gases.
6 Government response to the findings of the Overseer peer review report
• treats the soil as a single homogenous layer, limiting its ability to model soil hydrological
flows and, therefore, drainage of the soil solution and nitrate leaching
• does not adequately capture overland flow; such flow can, in some landscapes, be a
significant pathway for losses of non-nitrate forms of nitrogen. Overseer has focused more
on leaching of nitrate-nitrogen
• uses a form of climate data averaged over 30 years rather than actual climate data,
with the consequence that drainage and episodic weather events (eg, high rainfall
events associated particularly with high nutrient loss risk periods) are not adequately
represented.
The Government’s response to the Panel’s findings will be to put in place one or more of the
following options:
(a) the creation of a new risk index tool, potentially using elements of Overseer (including the
user interface); and
(b) development of a next generation Overseer to address the issues raised by the Review
Panel and ensure that it is fit for purpose as a tool to use in appropriate regulatory
settings; and/or
(c) greater use of controls on practices and inputs to manage nitrogen loss (including through
amendment to the NES-F); and/or
(d) a completely new approach to understanding and managing diffuse nutrient loss risk. This
might include, for example;
(i) near real-time monitoring of water quality at the local scale
(ii) a tool that provides detailed understanding of nutrient loss risk based on the
characteristics of land
(iii) a new nutrient loss model.
The options recognise that Overseer currently plays many roles and a wider suite of tools are
likely necessary to adequately fulfil future needs.
The options also recognise that some issues of concern identified by the Panel may be able to
be addressed. Some issues may be easier to address than others. For example, it would be
possible to modify Overseer to use actual climate data rather than data averaged over 30
years. Similarly, in most catchments with a high nitrogen load, nitrate-nitrogen is the
predominant nitrogen species. Hence the issues raised by the Panel that Overseer focuses on
nitrate-nitrogen, while not accounting adequately for nitrogen lost by overland flows may not
be material in those catchments and could be addressed by restricting the recommended use
of Overseer to particular catchments or situations (that is, where overland flows are not a
factor because of low slope and good drainage).
The Government has committed to continuing to support Overseer while the issues raised by
the Review Report, and possible options to address those issues, are resolved. This will ensure
continuity of access to Overseer, as required by some existing regional plans and resource
consents. It will also preserve options for the continued use and development of Overseer (or
elements of Overseer) as set out in (a) and (b) above, while considering further options (c) and
(d).
Statistical testing (such as sensitivity and uncertainty analyses) of all future nutrient
management tools will be considered.
Government response to the findings of the Overseer peer review report 7
In the immediate term, regional councils will continue to implement their plans and administer
consents to manage fresh water at the farm level. Councils will use all available evidence and
will need to be careful not to rely solely on Overseer modelling for their understanding of total
nitrogen losses. This will be particularly important where nitrate-nitrogen is not the dominant
form of nitrogen loss. Where existing plans provide for alternative modelling approaches or for
control to be exercised using means not dependent on Overseer, those opportunities should
be considered.
As regional councils continue their regional plan development consistent with their obligations
under the NPS-FM 2020, they can continue to use Overseer (as necessary) as an input to
catchment-level modelling subject to the due diligence checks as outlined in this report.
Similarly, councils can proceed to develop plans on the basis that farm-level nutrient loss
estimating and/or risk assessment tool(s) will be available for use for the implementation of
those new plans as required by the end of 2024, to give effect to the NPS-FM.
The Government will be taking action over the coming months to ensure all users are well
placed and can act with confidence. MfE will issue guidance material for regional councils on
the appropriate use of Overseer in its current form.
More broadly, MfE is also developing best practice guidance for models used in environmental
regulation. This will inform the approaches and tools developed and used in the longer term.
Transparency will be an important feature. This means the underlying scientific principles,
sources of data and equations used to build the models must be available for public review.
Transparency is vital for users and regulators to have confidence in models used in regulation.
8 Government response to the findings of the Overseer peer review report
1 Introduction
This report sets out the Government’s response to the whole-model review of Overseer (the
Review Report). That Review Report should be read alongside this document.
The Review Report was prepared in response to the 2018 report of the Parliamentary
Commissioner for the Environment (PCE) entitled Overseer and regulatory oversight: Models,
uncertainty and cleaning up our waterways.3 The PCE’s report noted that the whole Overseer
model, and many of its sub-models, had never been subjected to the rigour of independent
peer review and highlighted that a “significant amount of information needed to confirm
Overseer’s use in a regulatory setting is lacking”.
Accordingly, one of several recommendations made by the PCE was that an independent
review of the model be completed. In response, the Government convened an expert Science
Advisory Panel (the Panel) comprising local and international environmental modelling and
science experts.4
The Panel has now completed its review and its report has been publicly released. The Review
Report reached conclusions that may be of concern to many users of Overseer.
The purpose of this report is to set out how the Government intends to respond to the Review
Report and address the concerns of Overseer users to ensure agricultural producers and
councils have the tools needed to make sound land-use decisions and contribute to the
effective management of freshwater.
3 Parliamentary Commissioner for the Environment. 2018. Overseer and regulatory oversight: Models,
uncertainty, and cleaning up our waterways. Wellington: Parliamentary Commissioner for the
Environment.
4 The names and credentials of the Review Panel members are set out in appendix A.
Government response to the findings of the Overseer peer review report 9
2 About Overseer
Overseer is a computer software tool that models the nutrient flows onto and off farms and
farm blocks. Overseer aims to provide a quantitative description of farm nutrient dynamics for
a range of farm-system types. It models nitrogen, phosphorus, sulphur, potassium, calcium,
sodium and magnesium, as well as greenhouse gas emissions. It has supported farm decision-
making for many years. Overseer originated in the 1980s as a way to help farmers and their
advisors to make decisions about nutrient management and fertiliser use, in particular. As the
model has been developed over time, the uses to which it has been put have increased, as has
the variety of users and functions within the modelling tool.5
Overseer consists of the modelling software itself (not all of which is publicly accessible for
scrutiny by third parties) and an award-winning user interface that is regarded as easy to use.
The wider Overseer ‘ecosystem’ also includes a large database of farm input data and
modelling results allowing reporting of not just individual farm-nutrient management
performance but collective performance of multiple farms at various scales. This is a valuable
resource for nutrient management and for understanding nutrient-loss risk across the
landscape.
The Overseer model has various ‘sub-models’ within the Overseer model that represent
aspects of the nitrogen cycle within a farm system. These sub-models are built on a significant
body of scientific research, and that base research remains valid.
2.1 How Overseer is used The Overseer tool is used in many ways by different user groups. It is used by farmers and their
advisors as a tool to guide farm planning and management. Since around 2005, however,
Overseer has also been used by many regional councils to help in freshwater management and
planning. Use in regulatory contexts has been growing as regional councils have sought more
quantified control and accounting of diffuse agricultural discharges to fulfil their obligations
under the National Policy Statements for Freshwater that existed in the 2011–2019 period.6
The Overseer model is managed by Overseer Ltd. The Overseer model intellectual property is
jointly owned by the Ministry for Primary Industries, the Fertiliser Association of New Zealand
and AgResearch (the Owners) and is exclusively licensed to Overseer Ltd. A full discussion of
the ownership structure can be found in the PCE report.
2.2 Positive features of Overseer Positive features of Overseer that increase usability for farmers and their advisors include:
• automatic use of datasets (for example, climate and soil data)
• the use of real production information, feed and fertiliser input data, and farmer-
estimated crop yields. This removes the need for the model to predict information that
users already know
5 Retrieved from www.overseer.org.nz. 6 A description of the most common regulatory and non-regulatory uses of the tool is provided in
appendix B.
10 Government response to the findings of the Overseer peer review report
• an enabling user interface, providing a level of user comfort in interacting with Overseer
that is more difficult in many other models of agricultural and/or biophysical processes.
2.3 Policy context for Overseer use in regulation In 2020, the Government released the Essential Freshwater reforms. The reforms had three
main objectives:
• to stop further degradation
• to make material improvement in the health of waterways within five years
• set Aotearoa New Zealand on a path to restore its fresh water within a generation.
Significant components of the reform included the National Policy Statement for Freshwater
Management (NPS-FM) 2020 and the Resource Management (National Environmental
Standards for Freshwater) Regulations 2020 (NES-F).
As discussed below, the implementation of both instruments requires a tool (or variety
of tools) to be available to primary producers and regulators to assess the risk of nutrient
(and other diffuse contaminant) loss associated with land use. Being able to reliably estimate
nutrient losses from agricultural landscapes at both farm and catchment scales allows for a
broad range of regulatory responses and is particularly helpful when seeking to manage down
losses fairly and consistently at the catchment scale.
Despite that, the use of Overseer is not a mandatory requirement of either the NPS-FM or the
NES-F. Regional councils and others are not constrained in the tools they may use to fulfil
functions and responsibilities under these instruments. Regional councils and agricultural users
and their advisors would, nonetheless, be expecting to use Overseer for implementation of
some elements of those instruments.
The Review is important in that context. If Overseer is to be used to promote the Government’s
objectives for fresh water, insofar as they are affected by nutrients, the Government needs to
have a high level of confidence in its modelling estimates.
Government response to the findings of the Overseer peer review report 11
3 Overseer whole-model review
3.1 How the Science Advisory Panel undertook its review
The Panel was convened virtually in late March 2020 and met online many times over the
following 12 months. Panel members received documentation from AgResearch and Overseer
Ltd (including material that was the owners’ intellectual property and not in the public
domain); engaged in discussions about the modelling information with staff from both
organisations; and heard from users of the model, including representatives from Waikato
Regional Council and Environment Canterbury.
The Panel’s approach to the review represented international best practice for assessing
model adequacy, which includes assessing model structure, data used in model development,
model behaviour, model sensitivity and comparing model results with experimental data.
Much of the Review Report focused on a comprehensive assessment of the structure of the
Overseer model, to determine whether Overseer adequately simulates the main pathways and
processes that affect nutrient loss from agricultural farming systems.
Although the Panel acknowledged that assessments of model adequacy may include an
assessment of whether the model is in good agreement with observational and experimental
data, it also noted that a model’s underlying structure must be sound for such a comparison
with experimental data, to be useful. Therefore, the Panel did not consider comparisons with
such data as part of its review. The Panel preferred instead to focus on a detailed interrogation
of the model structure, as noted above. It considered that “comparing a large agroecosystem
model to measured observational or experimental data is a tremendous challenge, one not
commonly attempted in literature”. The reasons for this opinion are not repeated here but are
clearly set out in section 4.2.5 and section 10.2.5 of the Review Report.
The Panel drew on mātauranga Māori,7 assessing the extent to which Overseer can support
kaitiaki to improve or sustain the mauri and mana of Papatūānuku (the Earth Mother), as well
as modelling, agricultural sciences and nutrient dynamics.
Under its terms of reference, the Panel did not consider Overseer’s modelling of greenhouse
gases (see Kelliher et al (2015) and de Klein et al (2017) for reviews of Overseer’s greenhouse
gas components8).
The Panel did not assess the model for ease of use, its data files nor its use as a social tool to
encourage farmers to adopt farm management strategies that would successfully reduce a
catchment’s nutrient load.
7 Traditional concepts of knowledge and knowledge systems
8 Kelliher F, Rollo M, Vibart R. 2015. Desk-top review of GHG components of Overseer: Draft report
prepared for the New Zealand Agricultural Greenhouse Gas Research Centre. AgResearch report
RE500/2015/081. Hamilton: AgResearch. De Klein C, van der Weerden T, Kelliher F, Wheeler D, Rollo N.
2017. Initial review of the suitability of Overseer Nutrient Budgets Model for farm-scale greenhouse gas
reporting: Final report for the Ministry for Primary Industries and the Biological Emissions Reference
Group. AgResearch report RE450/2017/022. Hamilton: AgResearch.
12 Government response to the findings of the Overseer peer review report
3.2 Review findings The findings of the Panel are summarised below.
Overseer’s structure and data
The Panel found that the use of average climate data, homogeneous soil profiles and the
omission of ammoniacal nitrogen and soil organic matter dynamics within Overseer meant its
predictions do not account for potentially significant components of nutrient losses, at least in
some catchments and for some land uses. In addition, it found that Overseer does not, and
was never intended to, model episodic events, such as intermittent heavy rain or periods of
drought. The Panel considered this to be a significant weakness as episodic events can be
critical drivers of nutrient losses on some properties, and farm plans to manage nutrient loss
should factor such events into mitigation strategies. The Panel was also concerned that
Overseer does not balance mass9 and it inadequately accommodates deep-rooting plants.
Relative and absolute nitrate-loss estimates
The Panel noted that a common belief among users has been that using Overseer’s modelled
outputs to assess change or compare scenarios on the same property (that is, using it in a
relative sense) means absolute accuracy is not required. It advised, however, that Overseer is
unlikely to be a reliable tool for predicting either relative or absolute loss estimates. The
reasons given are found throughout its report and summarised in the executive summary of
the Review Report (page 7).
Mātauranga Māori and Te Tiriti o Waitangi
The Panel considered mātauranga Māori and Te Tiriti o Waitangi (the Treaty of Waitangi), and
made two important points.
First, Māori have a definitive relationship with the natural world, determined through
whakapapa. This remains crucial to environmental management, including of soil and water
resources, especially in production systems. This relationship forms the foundation of
mātauranga, which, in turn, informs cultural decision systems. Mātauranga is primarily
applied through kaitiaki responsibilities so that resource sustainability is both understood
and achieved.
Second, Te Tiriti o Waitangi and its related principles have a role in land and water decision-
support tools. Both mātauranga Māori and the principles of Te Tiriti o Waitangi must be given
effect in regulatory and decision-support tools for them to be fit for purpose for Māori.
The Panel then considered the Overseer tool itself in light of its above conclusions. It found
that, throughout the development of the Overseer model, there had been no engagement
with Māori. Neither the relationship Māori have with the natural world nor the role of Te Tiriti
9 Overseer balances the movement of nutrients between blocks, but this is not a guarantee that mass
balance is preserved for actual nutrient dynamics in crops and pastures. Over the duration of a model run,
the amount of nitrogen initially in the system plus the amount added may not equal the amount at the
end of the run plus the amount removed. The model uses the terms ‘balancing error’ and ‘nitrogen deficit’
to correct the numerical inequality in the model, but the Panel considered that this does not restore mass
balance in the formal sense of the process.
Government response to the findings of the Overseer peer review report 13
o Waitangi and its related principles have been included within Overseer or contributed to its
development. The Panel noted that Overseer Ltd has begun engaging with Māori more
recently, but for a model to be used in regulation in Aotearoa New Zealand, developers must
engage with and learn from Māori from the beginning, to ensure the model reflects te ao
Māori (the Māori worldview), incorporates mātauranga and is useful to Māori land users.
3.3 Panel’s conclusions The Panel summarised its core concerns with Overseer are that it:
• is a steady state model attempting to simulate a dynamic, continually varying system
• uses monthly time-steps
• uses average climate data and, therefore, cannot model episodic events or capture
response to climate variation
• does not balance mass
• does not account for variation in water and nutrient distribution in the soil profile
• does not adequately accommodate deep-rooting plants
• focuses on nitrate and omits ammoniacal nitrogen and organic matter dynamics
• lacks consideration of surface water and nutrient transport, as well as critical
landscape factors.
Because of these concerns, the Panel did not have confidence that Overseer’s modelled
outputs can inform whether changes in farm management reduce or increase the losses of
nutrients from the wide variety of property types and land uses across the country, or what
the magnitude or error of these losses might be. The Panel considered that:
Overseer’s structure is not adequate to provide more than a coarse understanding of
a farm’s nutrient losses (except for surface flows since these are not included in the
model). It also cannot reliably estimate how changes in farm management would affect
those losses.
As set out in appendix B, there is widespread use of Overseer in Aotearoa New Zealand.
Investment in the use and development of the Overseer model by different parties over recent
years has been substantial. Many stakeholders will consider that the various roles Overseer
plays are important to Aotearoa New Zealand’s social, cultural, economic and environmental
wellbeing. Additionally, regional councils, research entities and others managing freshwater
have a clear need for tools to help them understand the scale and movement of nutrients
across agricultural landscapes if the Government is to achieve its aspirations for fresh water.
For those reasons, the Panel’s findings raise serious concerns.
14 Government response to the findings of the Overseer peer review report
4 The Government’s response
The Review Report has potential implications for:
(a) the immediate ongoing implementation of regional councils’ planning and regulatory
functions under the Resource Management Act 1991
(b) the design and development of new regional planning provisions, to give effect to the
NPS-FM 2020 and to be notified by 2024
(c) the longer-term needs of all Overseer users
The following section sets out the Government’s response to (a) to (c) above.
4.1 Ongoing implementation of planning and regulatory functions
When considering the immediate issues, it is important to note that the Government has
committed to support Overseer (subject to regular reviews of progress) to ensure that the
model remains available for use as required by existing plans and consents (and to preserve
options for model redevelopment as discussed further below).
There is an important distinction between the use of Overseer to model nitrate leaching and
the use of Overseer to model loss of all species of nitrogen. Although the Review Report
expresses a lack of confidence of Overseer’s nitrogen modelling generally, many of its concerns
are based on the model’s focus on nitrate-nitrogen and inadequate representation of overland
flows and drainage. This suggests that where the dominant form of nitrogen loss is not nitrate
leaching, councils will need to use all available information and be careful not to rely solely on
Overseer modelling for their understanding of total nitrogen losses.
Existing resource consents and applications yet to be made
Overseer is embedded in the daily work of many regional councils and resource users
(particularly resource consent holders in catchments subject to nitrogen-leaching limits).
In some cases, operative regional plan rules already require resource consents with Overseer
‘N-loss’ numeric limits for various rural land uses and associated diffuse nutrient discharges.
Many consents have already been granted. Others are being applied for (or soon will be)
by applicants. Those existing resource consents must be administered and new applications
processed by councils as the consent authority. The Review Report does not change that.
It is important that regional councils continue to implement their plans, administer existing
consents and process new consents in a manner that promotes the objectives and policies of
those plans despite the issues raised by the Panel’s report. The Government is aware that
there are challenges in doing so and will work with the most affected regional councils to
develop guidance material. Some material will be generically applicable to all regional councils
that use Overseer in a regulatory context but some will be bespoke to specific councils,
recognising the particular characteristics and requirements of individual regional plans.
While the detail of that guidance is to be determined in collaboration with regional councils, it
will focus on the following general principles.
Government response to the findings of the Overseer peer review report 15
• Where opportunities exist to use additional data, alternative modelling or for control to be
exercised using means not dependent on Overseer, those opportunities should be
considered in addition to, or as an alternative to, Overseer modelling. This will be
particularly important where landscape factors and other evidence suggests that nitrate-
nitrogen is not the dominant species of nitrogen loss at the farm and/or catchment scale.
• As discussed in section 5, it is likely that Overseer could be redeveloped to improve the
confidence we can have in its nitrate loss modelling; accordingly, consents issued should
provide for the future potential use of a redeveloped model to the extent possible given
the scope of existing plan provisions.
• In the period before any redevelopment of Overseer, councils are legally obliged to
implement the provisions of their regional plans and resource consents. These statutory
documents may not allow alternative approaches to be adopted. However, to the extent
that existing plans and consents allow:
o Councils should use all available tools/data and be cautious in placing too much
emphasis on Overseer modelling as a means of exercising control through the
imposition of consent conditions or in any compliance and enforcement context.
Conditions placed on any new consents should allow for compliance to be
demonstrated using ‘multiple lines of evidence’
o compliance to be demonstrated with existing plans and consent conditions using a
‘best information available’ approach should be considered in addition to Overseer
modelling where plans and consents allow for such an approach. In that regard, the
body of science on which Overseer is, in part, based will be relevant to consider and
apply as appropriate to the context10
These principles are considered to be consistent with treating Overseer modelled estimates as
providing a coarse representation of nitrogen leaching only.
Administering existing resource consents after a new tool becomes available
When a new tool (or tools) becomes available (see section 5), consents may need to be
updated to ensure those tools can be applied and consents can be complied with, monitored
and enforced. While reviews of consent conditions under section 128 of the Resource
Management Act 1991 are an option, a more efficient and effective mechanism may be
required. This will be a matter that is considered as part of the resource management
legislative reform programme.
Implementing the National Environmental Standards for Freshwater
The NES-F regulate, among other things, specific forms of agricultural intensification and the
application of synthetic nitrogen fertiliser.
10 Nitrogen risk factors are well researched. This includes, for example, the relationship between livestock
urine patches (and hence stocking rates) and nitrogen leaching, or the relationship between nitrogen
fertiliser use and potential nitrogen losses (if farm production remains constant). Metrics, such as
purchased nitrogen surplus and nitrogen-use efficiency, will remain important and useful, particularly
when used in conjunction with an understanding of changes in nitrogen-loss risk factors (such as stocking
rates and fertiliser use).
16 Government response to the findings of the Overseer peer review report
The NES-F state that where a consent is required for agricultural intensification, consent will
only be granted if the applicant can show that the activity will not result in an increase in:
• contaminant11 loads in the catchment, compared with the loads as at the close of
2 September 2020
• concentrations of contaminants in fresh water or other receiving environments (including
the coastal marine area and geothermal water), compared with the concentrations as at
the close of 2 September 2020.
Similarly, where an applicant seeks consent to apply more than 190 kilograms of nitrogen per
hectare per year (kg N/ha/yr) they need to show that the amount of nitrogen leaching to water
would be no greater than would occur if the 190 kg N/ha/yr limit was met.
Although not specifically required by the NES-F, an Overseer report and nutrient budgets
comparing the ‘before and after’ scenarios would generally have been expected to support
applications for land-use intensification or for those exceeding the 190 kg N/ha/yr fertiliser
limit.
Given the Review Report findings, the use of the Overseer model alone for this purpose will
not provide sufficient certainty that the relevant requirements of the NES-F are satisfied. As
discussed in the context of resource consents under regional plans, additional evidence based
on accepted science will be needed to corroborate any Overseer modelling where used in
association with applications made under the NES-F. Once Overseer has been redeveloped or
an improved alternative tool is available, these should be used in preference to the current
Overseer model. Further detail on how applicants might approach the provision of such
evidence will be set out in specific advisory notes for regional councils provided by MfE.
4.2 Developing regional plans to meet the NPS-FM deadlines
Catchment modelling for 2024 regional plans and plan changes
To meet the 2024 deadline for developing new plans or modifying existing plans to give
effect to the NPS-FM, most regional councils have started or completed catchment-modelling
processes or will do so within the next 12 months. Some councils will have used or would be
planning to use Overseer modelling as part of these processes. This may be to understand
the sectoral source of contaminant loads and the cumulative effects in catchment loads of
different farm-scale mitigation options.
The availability of alternatives to Overseer in catchment modelling (for example, APSIM)12 is
understood to be severely limited because of capacity and capability constraints, because the
use of these models requires specialist expertise.
11 The Resource Management Act 1991 provides a broad definition of contaminants. In practice, however,
the contaminants of focus in these regulations are nitrogen, phosphorus, sediment and Escherichia coli (as
an indicator of microbial contamination).
12 APSIM: the Agricultural Production Systems sIMulator; for more information, see www.apsim.info.
Government response to the findings of the Overseer peer review report 17
Continued use of Overseer to inform catchment modelling and to develop plans in the
pre-2024 period will be necessary. This use is considered an appropriate interim approach
because catchment modelling:
• involves a high level of uncertainty
• does not require the same level of granularity in relation to nutrient-loss estimates as is
required for property-scale limit-setting or consenting purposes
• will not (necessarily) directly translate to regulated nitrogen-loss limits at the property
scale.
Even so, when deciding whether or how to use Overseer to inform catchment modelling and to
develop or amend plans before 2024, councils may need to consider the following:
(a) Whether there are other models or data platforms that can generate the required farm-
level nitrogen loss information, as an alternative to or to strengthen Overseer simulations,
in the timeframes in which it is required.
b) Whether Overseer estimates can be subjected to expert review and sense-testing using
multiple lines of evidence (for example, augmented by other models and/or expert
judgment) to build confidence in modelling outputs before they are used in catchment
modelling.
c) Whether the level of uncertainty associated with Overseer model estimates is acceptably
low given:
(i) the drainage characteristics of the land and the dominant form of nitrogen in the
catchment
(ii) the end use of the catchment modelling results.
Availability of fit-for-purpose tool(s)
The Government is committed to having a reliable and practical tool or tools to ensure regional
councils and resource users can effectively manage nutrient discharges as part of their new
regional plans.
The Review Report is not a reason to delay progressing regional plan development so as to be
ready for notification by the end of 2024. As noted elsewhere in this report, we aim to have fit-
for-purpose tools for use within regional plan rules be available within 12 months.
4.3 Managing the risks of increasing nitrogen discharges As councils reduce their emphasis on Overseer modelling for compliance and enforcement of
plan provisions and resource consent conditions, the risk is some resource users may perceive
an opportunity to increase farming intensity before a new tool is available to use (see section
5).
The advisory notes to be prepared by MfE and regional councils will outline how this risk
should be managed. These notes will focus on enforcing other non-Overseer controls (such
as conditions that limit inputs or address specific farm practices) either directly or through
farm environment plans, as far as possible within existing plan provisions and consent
conditions, and by clearly communicating that no resource user will benefit in the longer term
from short-term increases in intensity.
18 Government response to the findings of the Overseer peer review report
The need to ‘measure’ and enforce any such increases in intensity will be a consideration in
the design of the tool(s) (see section 5).
The Government will monitor this risk closely. If necessary, an amendment to the existing
NES-F could be made along with amendments already planned. Such an amendment would
target potential intensification of existing farming systems in catchments subject to Overseer-
reliant rules and would ensure regional councils have broad scope to impose appropriate
consent conditions, despite any limitations imposed by their existing plans. Any such change
to the NES-F would benchmark affected farms to a date no later than the date on which this
report is released.
Government response to the findings of the Overseer peer review report 19
5 The way forward: the next 12 months
In the immediate term, the Review Report has implications for the degree of reliance that can
be placed on modelled estimates from Overseer in its current form, in the context of existing
regulatory uses of the model. The Review Report means the Government needs to assess the
current approach to managing nutrients, reassess what tools (or range of tools) are really
needed, and investigate the options for meetings those needs.
Since regional councils began focusing on addressing diffuse discharges of nutrients (during
the past 10–15 years), Overseer has been the only tool readily available to them to manage
nitrogen-leaching risk in a quantified way. It has been used as a policy-making and policy-
implementing tool and for property-scale and catchment-scale nutrient management. It
has been used in catchments with severe nitrogen enrichment issues and in catchments
where nitrogen issues are of lesser concern. It has been used, in some fashion, by most
regional councils.
Yet the current and future needs of regional councils and other potential users vary greatly.
This depends on, among other things, the extent that nitrogen discharges and, specifically,
which forms of nitrogen pose most risk to freshwater outcomes, the existing concentrations
and forms of nitrogen in receiving water bodies, and the nature of land uses in a catchment.
It is clear a broader suite of management tools is desirable.
At the same time, the Government recognises that a tool(s) to allow for the regulation of
quantified property-specific reductions in nutrient discharges remains important in catchments
with major nutrient-related freshwater quality challenges. Similarly, models that can estimate
nutrient-loss at the farm-scale will be important if we are to ‘calibrate’ new nutrient loss risk
management tools (such as risk indices; see appendix C) and/or estimate the effect on
catchment nutrient loads of other, more direct, forms of regulation (for example, input
controls).
Accordingly, within the next 12 months, the Government will investigate, and put in place, one
or more of the following options13
(a) the creation of a new risk index tool, potentially using elements of Overseer (including the
user interface); and
(b) development of a next generation Overseer to address the issues raised by the Review
Panel and to ensure that it is fit for purpose as a tool to use in appropriate regulatory
settings; and/or
(c) greater use of controls on practices and inputs to manage nitrogen leaching (including
through amendment to the NES-F); and/or
(d) a completely new approach to understanding and managing diffuse nutrient loss risk. This
might include, for example:
(i) near real-time monitoring of water quality at local scales
(ii) a tool that provides detailed understanding of nutrient loss risk based on the
characteristics of land
13 The options referred to in (d) are likely to be longer-term possibilities than options that would be ready to
implement within 12 months.
20 Government response to the findings of the Overseer peer review report
(iii) a new nutrient loss model.
For those reasons, the work streams set out in sections 5.2 to 5.4 are proposed.
5.1 Ongoing support for regional councils As noted, the outcomes of the Review Report pose significant challenges for regional
councils. The Government has prepared guidance for regional councils on using Overseer in its
current form and covering the matters likely to be affected. These include consenting, regional
plan-making, and compliance, monitoring and enforcement.
The Government will also continue to support the councils most directly affected. Further
guidance material will be developed as needed.
5.2 Understanding our current and future needs The Government proposes to use the next 12 months to further investigate the options
identified above and develop the preferred option(s) so they are available by mid-2022.
This will begin with a needs analysis to ensure the Government has a clear and common
understanding of the needs of all user groups. This work has already started.
Following this analysis, it is anticipated that, within a 12-month timeframe, there will be:
• a clear vision of Aotearoa New Zealand’s nutrient-management tool needs at national,
regional and local-user scales
• a tool(s) available for immediate use to replace and/or complement sole reliance on
Overseer (being the risk index tool and, potentially, a redeveloped Overseer)
• a commitment to the development of additional tools over the longer term should those
be necessary to meet our future needs.
Advice received during the review process regarding the Māori concepts and principles to be
applied to this work will inform the work programme. This will include working with iwi and
Māori on developing and implementing the programme.
The proposed work programme for the next 12 months will consist of three core workstreams,
as described below and set out in figure 1.
All tools developed under this programme will be subject to sensitivity and uncertainty testing
to the extent that is feasible and likely to yield reliable results.
At a broader level, an important parallel project is under way to better understand the
expectations for, and uses of, models in environmental regulatory settings generally in
Aotearoa New Zealand. MfE has commissioned guidance on this matter. That guidance is
expected to be completed in late 2021.
5.3 Option (a): Nutrient risk assessment approach MfE is leading the development of a risk assessment framework as an alternative and/or
complementary tool to farm-scale nitrogen-loss modelling. Although described here as
Government response to the findings of the Overseer peer review report 21
an option, a commitment is in place to proceed with this tool development now, ahead of
the needs analysis, to ensure its early availability.
The role this new tool will play in managing diffuse nutrient discharges is yet to be determined
but it will be capable of fulfilling at least some of the roles currently performed by Overseer.
MfE aim to have it available for use within 12 months.
Risk assessment and management approaches have been used as a means of managing
a range of contaminants in both Aotearoa New Zealand and overseas. They have been
developed based on accepted nutrient management science and have successfully reduced
contaminant losses from farms in several jurisdictions. Examples are discussed in appendix C.
Although the most common risk indices used relate to phosphorus (see below), nitrogen
risk indices have been developed where diffuse nitrogen discharges contribute to water
quality deterioration. Risk indices have been developed to ensure that land management
practice improvements are undertaken to reduce contaminant losses but, critically, these
improvements would be effective at addressing the relevant issue in a specific receiving
environment. In other words, to ensure that changes are targeted at the ‘riskiest’ practices in
the ‘riskiest’ areas, so that the changes implemented are the most effective based on those
risk profiles.
Contaminant risk assessment frameworks can take several forms. These include:
(a) a simple risk index that considers and ‘scores’ risks associated with either the:
(i) choice of farm practices and inputs; and/or
(ii) basic biophysical conditions within which the farm operates (soil type and rainfall)
along with farm system type; or
(b) a more sophisticated assessment that, in addition to both the matters above, considers
wider biophysical risk factors, for example, slope, aquifer characteristics and the
sensitivity of the receiving environment.
The risk assessment can take the form of a tabulated scoring system or a more sophisticated
approach using software that can integrate and weight an array of factors to produce an
indexed risk rating.
A critical factor of the risk assessment approach is that, for the process to improve practices,
it must be coupled with an implementation mechanism. The risk assessment process itself can
provide useful information about a farm’s levels of risk, but a means is needed of ensuring that
actions are taken to reduce that risk to ensure environmental outcomes are met.
For example, risk score thresholds could be used as a trigger for activity status in regional plans
(as Overseer leaching rates have been), or the risk-index approach could be embedded within
resource consent conditions or mandatory farm environment plans to drive practice change
or audit gradings.
One potential benefit of a risk assessment regulatory approach is it could be used to
address risks associated with loss of a wide range of diffuse contaminants beyond
nitrogen (phosphorus, sediment and Escherichia coli), although this would add complexity.
22 Government response to the findings of the Overseer peer review report
5.4 Option (b): Amending Overseer to address Panel concerns
As set out in section 5.2, the option of developing a next generation Overseer remains and a
redevelopment project is being established. It would appear that at least some of the issues of
concern identified by the Panel may be able to be addressed. Some of these may be easier to
address than others, such as modifying Overseer to use actual climate data rather than 30-year
averaged data. Similarly, the issues raised by Overseer focusing on nitrate-nitrogen and not
accounting for overland flows could be addressed by restricting the recommended use of
Overseer to particular situations (for example, where nitrate-nitrogen is the predominant
nitrogen species and slope and drainage characteristics are not conducive to overland flows).
In most catchments with greatest nitrogen load, nitrate-nitrogen is the predominant nitrogen
species. Nevertheless, non-nitrogen species can be important contributors to freshwater
degradation in some catchments. Hence the use of even a redeveloped Overseer may need
to be supplemented with additional tools.
Importantly, when Overseer has been evaluated against farm systems experiments, and
when actual climate and drainage data are used to parameterise the model, relatively good
agreement has been reached between Overseer’s output and measured nitrate leaching
losses. This suggests that if Overseer can be ‘redeveloped’ to use real climate data and to
adequately model drainage, users could have greater confidence in its use for estimating
nitrate leaching.
Modelling overland flow and the non-nitrate species of nitrogen is more difficult. A critical
needs analysis will be undertaken with regional councils. This will seek to understand in what
catchments overland flow is a significant contributor to declining water quality and, where it is
a concern, whether ammoniacal and organic nitrogen losses can be addressed in ways similar
to how phosphorus and sediment are dealt with now.
Therefore, a redeveloped Overseer could continue to have a role. As confirmed by the Review
Report, however, a suite of tools will be required in addition to a redeveloped Overseer to
efficiently and effectively address all of our nutrient management needs for the future.
The Government will work to support the development of a next generation Overseer so that
it can fully investigate the opportunities and plan an appropriate path for redevelopment.
5.5 Option (c): Control of practices and inputs Controls are in place already at both regional and national levels on certain practices and
inputs that aim to manage the nutrient loss risk from farms. These include, for example,
regional council rules controlling the discharge of collected effluent, cultivation, irrigation
and earthworks. Similarly, the NES-F already controls the application of synthetic nitrogen
fertiliser and intensive winter grazing, and national regulations control stock access to
water bodies.
If maximum nitrogen leaching rates cannot be set because of an inability to reliably monitor
compliance, then another option is to extend the practice and input control approach.
This approach has well-known advantages and disadvantages that will need to be weighed
against the risks faced and the efficacy of other available options.
Government response to the findings of the Overseer peer review report 23
Practice and input controls can be at the farm scale (by way of freshwater farm plans and
individual resource consents), the regional or catchment scale (through regional plan rules),
or at the national scale (through the inclusion of additional standards in the existing NES-F).
As with the risk index tool, input and practice controls would best be calibrated using a model
(such as APSIM) to help us understand the actual and potential effect of the controls on
nutrient-loss at the property and catchment scales.
5.6 Option (d): New management tools The Government fully expects that further needs analysis may well identify the need for
additional, or perhaps alternative, tools for the effective and efficient management of nutrient
loss risk. At this stage, those alternative tools have not been fully scoped. The Government
will, however, be further investigating the feasibility of the following options, as a minimum.
(a) Near real-time monitoring of freshwater quality at high resolution (potentially farm) scale.
This technology and investment could allow, for example, property-scale nitrogen loss
limits to be imposed with compliance dependent on surface water monitoring rather than,
or in additional to, below the root zone modelling.
(b) Landscape risk assessment. Nutrient loss risk is dependent on land use, climate and
underlying physical characteristics of land. Understanding the vulnerability of land to
nutrient loss (and the main transport pathways of that loss) allows controls to be tailored
to maximum effect and action and investment to be prioritised.
(c) A new nutrient model. To enable the effective and equitable regulation of nitrogen loss
reductions, a nutrient loss modelling tool that can perform the function that some
regulators have sought from Overseer over the past 15 years may be identified by the
needs assessment as important. If Overseer cannot be restructured to perform that task, a
new model may need to be developed. This is a long-term and likely expensive option so
will only be considered in detail if it is determined that other options cannot deliver the
required level of risk management and control.
24 Government response to the findings of the Overseer peer review report
Figure 1: Future work timeline
2021 2022 2023 2024
July–Dec January–June July–Dec January–June July–Dec January–June July–Dec
Overseer
New tools
Policy development
Risk assessment approach
Other new tools
Regulations/ planning
framework
Existing plans, consents
NPS-FM plan dev. – catchment modelling
NPS-FM plan dev. –
limits setting
Future tools needs analysis:
• catchment modelling
• property scale
• on-farm decision tool
• monitoring
• allocation
• compliance
Future tools developed and decided and progressed as appropriate, based on future needs
Consents and plans valid – continue with implementation, compliance and monitoring
Risk assessment tool available to use in existing resource consent conditions (to replace Overseer numbers and rates)
Overseer may be used for catchment modelling (with provisos)
Options assessment and decisions for regulatory tools (including Overseer)
Tool and implementation mechanism(s) development
Risk assessment tool available. Property-scale risk index to be used for property-scale regulatory decisions and compliance
Risk assessment tool to be used for property-scale regulatory decisions and compliance
Risk assessment tool available for use in catchment modelling NPS-FM plans to be notified 31/12/24
Current version available
Future development pathway progressed, based on future needs, decisions and ownership agreement
Government response to the findings of the Overseer peer review report 25
Appendices
Appendix A: Science Advisory Panel
Dr Ian Johnson
Dr Johnson is a mathematician with experience in developing and writing biophysical
computer simulation models incorporating environmental physics, plant, crop and pasture
growth, soil hydrology, soil organic matter and nutrient dynamics, and animal growth and
metabolism. Early in his career he was with the Biomathematics Department at the Grassland
Research Institute in the United Kingdom and then the Department of Agronomy and Soil
Science at the University of New England in Armidale, New South Wales. More recently, as
Director of IMJ Consultants, he has developed models in collaboration with universities and
industry bodies in Australia and Aotearoa New Zealand. He is widely published in the scientific
literature, including as co-author of the textbook Plant and Crop Modelling (Thornley and
Johnson, 1990, 2000).
Mr Dave Clark
Mr Clark is a dairy industry and research consultant. Between 1991 and 2013, he was Principal
Scientist at the Dairying Research Corporation/Dexcel/DairyNZ. His research during that time
looked at the intersection of farm economics and environmental impact and was supported by
a philosophy that environmental protection and profitable dairy farming are not mutually
exclusive. In 2009 he was awarded the New Zealand Grassland Trust – Ray Brougham Trophy
for services to Aotearoa New Zealand farming systems. He carried out mainly hill country
research when he worked at Grassland Division, Department of Scientific and Industrial
Research in the early stage of his career.
Dr Brent Clothier
Principal Scientist with Plant & Food Research, Dr Clothier has extensive experience in soil
science, especially with the measurement and modelling of water and solute movement in soil.
He has published more than 300 peer-reviewed publications. He was elected a Fellow of the
Royal Society Te Apārangi in 1994 and was the President of the New Zealand Society of Soil
Science from 2008 to 2010. He is an Academician (Foreign) of the Chinese Academy of
Engineering (Agriculture Division).
Dr Donna Giltrap
Research Priority Area Leader for Agricultural Greenhouse Gases Emissions and Mitigation at
Manaaki Whenua – Landcare Research, Dr Giltrap is a modeller with a background in physics
and mathematics. Her doctorate is in physics and she also holds a graduate diploma in applied
statistics. She was part of the team that reviewed the nitrous oxide component of Overseer
in 2018. She is a member of the New Zealand Soil Science Society.
Dr Clint Rissmann
Dr Rissmann is the Founder and Director of Land & Water Science Ltd. He is also a Senior
Adjunct Fellow in the Waterways Centre for Freshwater Management – a partnership between
the University of Canterbury and Lincoln University. He has more than 10 years’ experience in
earth systems science, specialising in water quality, biogeochemistry, greenhouse gases and
systems thinking. He has co-authored several peer-reviewed publications researching soil and
water quality in Aotearoa New Zealand. He is a leading proponent of the physiographic
approach, which involves understanding water quality outcomes based on an integrated
understanding of landscape properties.
26 Government response to the findings of the Overseer peer review report
Dr Nick Roskruge
Dr Roskruge is of Atiawa ki Taranaki and Ngāti Tama-ariki descent. He is Professor in
Ethnobotany at Massey University and, since 2003, has been Chairperson of Tāhuri Whenua,
which represents Māori interests in the horticulture sector. He is a member of the Māori
Advisory Board for Resilience to Nature’s Challenges – a National Science Challenge – and
is also a member of the Hazardous Substances and New Organisms Committee of the
Environmental Protection Authority (EPA). Previously he was Chair of Ngā Kaihautū Tikanga
Taiao, the EPA’s Māori advisory committee. He holds a doctorate in soil science with his
doctoral thesis looking at Māori land development through traditional knowledge, and the
soil and horticultural sciences. He has spent time in Peru and Chile, where he worked on
crop genetics and indigenous systems projects. He received a Fullbright Scholarship in 2013,
undertaken at Cornell University (USA).
Dr Peter Thorburn
Dr Thorburn is a Chief Research Scientist and Research Group Leader in the Commonwealth
Science and Industrial Research Organisation (CSIRO) in Queensland, Australia. He is
responsible for agricultural systems research and is internationally recognised for his expertise
in crop systems modelling. He represents CSIRO on the Agricultural Production Systems
sIMulator (APSIM) initiative, which owns the APSIM advanced farming systems model, and is
co-lead for crop modelling in the international AgMIP program. He has extensive experience
in scientific advisory groups, including as a member of groups on managing water quality in
Great Barrier Reef catchments and reviewing or advising on Overseer in 2012 and between
2014 and 2017.
Dr Robin White
Dr White is Associate Professor of Integrated Beef Systems Management at Virginia Tech in
the United States of America. She is a member of the American Dairy Science Association, an
editor for the Farm Systems Analysis and Economics and Resources and Environment sections
of the Journal of Dairy Science, and an Editorial Board member of the Journal of Animal
Science. Her research focuses on leveraging data analysis and animal nutrition to enhance the
sustainability of food production systems. She graduated from Washington State University as
a Doctor of Philosophy in Animal Sciences.
Government response to the findings of the Overseer peer review report 27
Appendix B: Existing use of Overseer
Use of Overseer by regional councils
Overseer has been used to different degrees and in different contexts by regional councils
around the country. Some councils have Overseer embedded (whether explicitly or indirectly)
within their planning and consenting framework, while some use the tool as a non-regulatory
method to encourage good farm management practices modelled to reduce nitrogen-leaching
rates. Other councils sit along a spectrum between those two points and these various uses
are outlined below. Overseer is used in some form by more than half of regional councils in
Aotearoa New Zealand.
As a tool to help with catchment modelling
Overseer has been used to estimate nitrogen and phosphorus loads from various agricultural
and horticultural land, including:
• as inputs to catchment accounting models, including to identify of key nutrient-loss
source areas
• as a method of corroborating catchment accounting model outputs
• as a means of understanding the on-farm implications of modelled catchment loads,
limits and reduction targets
• in conceptual modelling, to develop ‘what-if’ scenarios around land-use change
(both intensification and de-intensification) and ‘bundles’ of options to mitigate
environmental effects.
In plan making and regulation design
Overseer-derived nitrogen-leaching rates have been used by councils when developing plans
and associated regulations to manage land-use intensification, including:
• in plans as trigger points for defining resource consent activity status (for example, farms
modelled to be leaching more than a specified rate of nitrogen are required to obtain a
resource consent rather than being a permitted activity)
• in plans as trigger points for the application of specific policy approaches upon consenting.
Limit setting (either in plans or resource consents)
The National Policy Statement for Freshwater Management 2020 (NPS-FM) (and its
predecessors) requires regional councils to set values, objectives and limits for freshwater
bodies in their region. Some councils have used Overseer as the means of setting limits and
managing freshwater discharges to achieve objectives. This has been through:
• the setting of property-scale nitrogen-leaching limits as absolute rates, for example, a
requirement that farms must not discharge more than 30 kilograms of nitrogen per
hectare per year, as modelled using Overseer
• the setting of property-scale leaching limits as rates relative to a baseline leaching rate,
for example, a farm must reduce its leaching rate over a period of five years by 10 per cent
from the average annual leaching rate during a specified period
• as an extension on the above usage, the setting of property-scale leaching limits within a
catchment-scale cap-and-trade system. In these instances, a total nitrogen load for a
28 Government response to the findings of the Overseer peer review report
catchment is set, with maximum leaching rates capped for individual properties. The caps
can only be exceeded if nitrogen is purchased from another land user whose leaching rate
is below the cap.
Consenting
Overseer has been used as a part of the resource consent processes by some councils as well
as within plans as:
• a method approved for determining nitrogen losses
• the means of allocating nutrient losses to farms operating under a collective nutrient
discharge allowance resource consent
• the means of demonstrating that an activity will not result in an increase in contaminant
losses (specifically nitrogen) when seeking to intensify land use, which requires a resource
consent under the Resource Management (National Environmental Standards for
Freshwater) Regulations 2020.
Compliance
Overseer has been used in some regions to demonstrate compliance with resource consent
conditions or permitted activity standards, such as the distribution of nitrogen loading across
multiple users, demonstrating nitrogen reductions, or demonstrating that nitrogen discharges
are not increasing beyond an approved baseline discharge limit.
Within farm environment plans
Farm environment plans (FEPs) will usually incorporate a range of management areas, such as
soil health, nutrient management, riparian management, irrigation, biodiversity and effluent
management. The nutrient management component will generally require the completion of a
nutrient budget, and Overseer is the most common method to derive these and is compulsory
under some programmes.
Freshwater farm plans (or FEPs with a freshwater component) will become mandatory in the
future as part of the Essential Freshwater policy package.
Freshwater accounting
Overseer has been used by some councils as a method within their freshwater quality
accounting systems, similar to its use in catchment modelling processes. This can include
tracking land-use change and intensification levels over time against a baseline state and
estimating further effects on nutrient loads and concentrations (such as under altered flow
regimes caused by climate change).
Overseer as a non-regulatory decision-support tool
Overseer is widely used across the primary sector for various non-regulatory uses. The use of
Overseer as an on-farm decision-support system continues to be its fundamental purpose.
OverseerFM, the current version of the tool, is used on thousands of farms across Aotearoa
New Zealand. This core function has extended beyond the farm gate, and the tool is now used
in a variety of settings for a variety of purposes.
The main uses of Overseer beyond those in the regulatory context are summarised below. This
is not an exhaustive list.
Government response to the findings of the Overseer peer review report 29
Some of these are related to the use of Overseer in the regulatory context, however, they are
not explicitly described above and are therefore set out here.
Nutrient budgeting
The Overseer nutrient budget is the main output of the tool. It describes how nutrients flow on
farm blocks or across the whole farm. A budget is broken down into nutrient added, removed
and internal transfers.
The budget allows a farmer or grower to see where nutrients are being lost to the
environment. Regulation has focused on Overseer’s modelling of nitrogen lost through
the root zone. Overseer, however, also models phosphorus, potassium, sulphur, calcium,
magnesium and sodium. Various factors (farm-system type, receiving environment, soil
properties and so on) will determine which nutrients are critical for a particular farm or block.
Greenhouse gas emissions recording and reporting
The Overseer model estimates methane, nitrous oxide and carbon dioxide emissions for a
farm by their source. Estimated values for each gas are converted to kilograms of carbon
dioxide equivalent per hectare per year so they can be compared with each other and
other greenhouse gas reporting. Greenhouse gas estimates have been available in
Overseer since 2008.
A carbon stock tool has also recently been added, which estimates the carbon sequestration
potential of tree blocks on farm.
Benchmarking and aggregated reporting
OverseerFM (launched in 2019) allows for farm benchmarking and aggregated reporting. This
is used by groups of users to assess their performance against other farms and can provide for
group reporting. This is particularly useful where farms operate under a collective
environmental management strategy or system.
Scenario testing and planning, investment decision support
Many farmers and growers use Overseer as a means of informing investment options on farm,
using the scenario-testing functionality. This is particularly the case where Overseer has been
used in a regulatory context and a fixed nitrate-loss number must be attained or a percentage
reduction in modelled losses is required over time.
Overlaying financial information onto these options means farmers can assess different
options to reduce their environmental footprint;– the best bang for their buck. These options
can then also be assessed by financial institutions (see below).
Other uses
Due diligence, banks and other lending institutions
Banks and lending institutions use Overseer to assess the bankability of options on a
farm within environmental limits, and as part of due diligence on assessing farm sales
and purchases.
30 Government response to the findings of the Overseer peer review report
Overseer output reports are also used in the marketing and saleability of properties,
particularly those with nitrogen limits imposed through regional regulations.
Some institutions may offer premium products to farmers able to demonstrate high levels of
sustainability (for instance, reduced interest rates on borrowings). Sustainable finance is a
rapidly growing trend across the globe, where financing is linked to sustainability targets
agreed by the lender and borrower. If the borrower meets the targets, they receive a funding
discount, but if their environmental indicators worsen, they may have cost increases imposed.
Use in research
Overseer has been used extensively in research in Aotearoa New Zealand to help in
understanding nutrient flows and farm environmental outputs.
Nutrient management qualifications and certification
Overseer is used within several university undergraduate programmes. It is also a key
component of Sustainable Nutrient Management in New Zealand courses run by Massey
University. Completion of these courses is required to become a Certified Nutrient
Management Advisor.
Component or contributor to other tools, models or systems
Overseer is used as a component of, or contributor to, other tools, models and systems.
Where Overseer outputs or reports are a direct component of the tool, the implications will be
more significant than if it is an indirect input.
Non-regulatory farm environment plan requirement
Processors and collective companies (such as irrigation schemes) often require their suppliers,
members or shareholders to undertake farm environment planning (see above) or incentivise
their use.
Government response to the findings of the Overseer peer review report 31
Appendix C: Examples of risk assessment approaches Regional council engagement has highlighted the need for any risk-index system developed to
be applicable across a range of land-use types, or for different indices to be designed to cater
to the different risks associated with different land-use types (for example, a risk index for
dairy and a different risk index for vegetable growing).
Two examples of nitrogen-risk indexing systems are provided below. These are provided for
illustrative purposes only and do not reflect government policy. The first was developed by
McDowell et al14 for use in the United States of America. The second was developed by
Fonterra as part of its Tiaki programme to help its shareholders manage nitrogen risks in the
New Zealand context. It is therefore designed specifically for the New Zealand dairy sector.
Example 1: Nitrogen-indexing system
Nitrogen-indexing system (from McDowell et al, 2002)15
Factors None (0) Low (1) Medium (2) High (4) Very High (8)
Part A
Nitrogen Transport Characteristics
Texture Clay Clay loam to
silty clay loam Loam to silt loam
Loamy fine sand to coarse
sandy loam Sand
Permeability N/A
Infrequent irrigation on well-drained
soils
Moderate irrigation on soils with slopes < 5%
Frequent irrigation on
soils with slopes of 2% to
5%
Frequent irrigation on
soils with slopes < 5%
A score = texture rating x permeability rating
Part B
Nitrogen Source Characteristics
Soil LRV = Soil Test P (mg P kg-1) x 0.2 x CP
Fertiliser rate = kg P ha-1
Fertiliser application method and
timing
Very low (0.2)
Placed with planter or injected
more than 2 inches deep
Low (0.4)
Incorporated < 1 week after
application
Medium (0.6)
Incorporated > 1 week or not
incorporated > 1 week following
application May–Oct
High (0.8)
Incorporated > 1 week or not
incorporated following
application Nov–Apr
Very high 1
Surface-applied on frozen or
snow-covered soil
Fertiliser LRV = Fertiliser rate x application method and timing score
Manure application method and
timing
Very low (0.2)
Placed with planter or injected
more than 2 inches deep
Low (0.4)
Incorporated < 1 week after
application
Medium (0.6)
Incorporated > 1 week or not
incorporated > 1 week following
application May–Oct
High (0.8)
Incorporated > 1 week or not
incorporated following
application Nov–Apr
Very high 1
Surface-applied on frozen or
snow-covered soil
Manure LRV = Manure rate x application method and timing score
B score = sum of source characteristic ratings
N Index Rating = A score x B score
14 McDowell R, Sharpley A, Kleinman J. 2002. Integrating Phosphorus and Nitrogen Decision Management at
Watershed Scales. Journal of American Water Resources Association 38, 2.
15 Ibid.
32 Government response to the findings of the Overseer peer review report
Example 2: Nitrogen-risk scorecard
The Tiaki programme scorecard assesses risk across six farm practice areas that have been
identified as the primary drivers of nitrogen loss from dairy farms. Each management practice
starts from a position of zero points, with points being added under each key risk practice.
Moderating practices can add or subtract points depending on the practice.
For example, in relation to wintering practices, break-feeding on fodder crops will add 40 risk
points to the base score, while wintering cows off pasture (such as in a barn or on a wintering
pad) will reduce the base score by 40 points. The scores for each practice area are then
totalled. All scores less than 20 are considered very low risk, and all scores greater than 80
are considered high risk.
Management practice Key risk practice Moderating practices
Nitrogen (N) fertiliser application
Amount of applied N
Conversion efficiency
Timing of application
Feed budget use
Application rate
Stock management Peak stocking rate
Pasture eaten
Replacement grazing
Wintering practices (x 2)
Imported feed Total N imported through feed
Ratio of N supplements to product
Average N content of imported supplements
Irrigation Irrigation type Soil moisture monitoring
Application methodology
Cropping and cultivation
Percentage of farm cultivated annually via minimum till
Season of harvest
Percentage of farm cultivated annually via conventional method
N fertiliser application
Effluent management Discharge method Application depth
Disposal area
A mock example of how the final scores are set on the risk scale is provided below.
Farm management practices
Risk ratings
Very low < 20
Low 20–39
Medium 40–59
High 60–79
Very high 80 <
Stock management
Nitrogen fertiliser
Imported feed
Cropping and cultivation
Irrigation
Government response to the findings of the Overseer peer review report 33
Nitrogen-risk indices in action: US Clean Water Act
In the United States of America, the federal Clean Water Act (CWA) established the basic
structure for regulating discharges of pollutants into waterbodies and regulated water quality
standards (across a range of indicators) for surface water. The key parts of the CWA came
into effect in 1972. At the national level, the CWA is implemented by the Environmental
Protection Agency.
The CWA made it unlawful to discharge any pollutant from a point source into navigable
waters without a permit. States were also required to identify waterbodies that were not
meeting water quality standards (for various outcome areas including recreation, ecosystem
requirements, drinking water and so on) set by the CWA (known as ‘impaired waters’) and
then implement programmes to improve water quality.
The main parameter for determining water quality limits under the CWA is the total maximum
daily load (TMDL). This is the maximum loading rate of given pollutants that a receiving water
can assimilate without resulting in water quality impairments. TMDLs are set at the watershed
(catchment) scale and include both point and non-point contributions. The TMDL approach is
now used across the United States of America. Where a water body is ‘impaired’, a TMDL
must be set to improve water quality to meet the relevant quality standard for the particular
use value.
Where non-point source pollution is identified as contributing to the exceedance of TMDLs
for impaired watersheds, most states have adopted environmental risk-index approaches to
improve land management practices. Risk indices have been developed at the national, state
and county levels across a range of contaminants.
A risk index for managing nitrogen in the United States of America will (at a minimum)
determine the risk profile of a particular farm based on environmental (soil and climate)
factors (see the national Leaching Index, below). Farms are then required to implement best
management practices (BMPs) based on environmental risk-factor rating and their farm-
system type. More sophisticated indices incorporate management practices within the index
itself, to determine the overall rating (see the case study: “Nitrogen risk scoring in Virginia’s
Muddy Creek”, below).
The United States Department of Agriculture’s Natural Resources Conservation Service (NRCS)
develops conservation practice standards, which state-level conservation practice standards
must comply with at a minimum. State conservation practices can be more stringent than
those set at the national level.
The NRCS Nutrient Management Code 590 applies to “all fields where plant nutrients and soil
amendments are applied” and specifically to nitrogen, phosphorus and potassium. It requires
NRCS-approved nitrogen-risk assessments to be undertaken “for all fields where nutrient
management is planned unless the state NRCS, in cooperation with state water quality control
authorities, has determined specific conditions where nitrogen leaching is not a risk to water
quality, including drinking water”.
The nationally mandated risk assessment criteria for nitrogen are either the Leaching Index or
the Nutrient Tracking Tool. The Leaching Index will provide a score for a field based on soil
type (including whether subsurface (tile) drainage occurs) and climate. The score for the field
will then determine the appropriate management practices.
34 Government response to the findings of the Overseer peer review report
Leaching Index Score Potential Leaching Technical Guidance
< 2 Low potential to contribute to soluble nutrient leaching below the root zone
None
> = 2 and < = 10 Moderate potential to contribute to soluble nutrient leaching below the root zone
Nutrient management (Code 590) should be planned
> 10 High potential to contribute to soluble nutrient leaching below the root zone
Nutrient management (Code 590) must be planned AND practices planned to trap soluble nutrients before field loss via surface or subsurface drainage
The Leaching Index is being replaced in some states by the Nutrient Tracking Tool. This is
an online tool that assesses nutrient-loss risks based on an array of environmental and
practice input data, at a highly granular level. The most effective BMPS (environmentally and
economically) can then be assessed using the tool as part of farm planning processes. A risk
score is provided in the form of estimated nutrient loss rate, with a plus–minus margin of
error. See https://ntt.tiaer.tarleton.edu/welcomes/new?locale=en for more information.
Government response to the findings of the Overseer peer review report 35
CASE STUDY: NITROGEN RISK SCORING IN VIRGINIA’S MUDDY CREEK IMPROVES WATER QUALITY
The Muddy Creek watershed in Virginia was found to be impaired for nitrate-nitrogen in
1998 because part of the river is designated for public drinking water and drinking water
standards (10 milligrams per litre) were being breached. The total maximum daily load (TMDL)
for nitrate-nitrogen was determined in 2000. Sources at the sub-catchment level were
identified and the main sources of nitrogen were failing septic tank systems, pipe losses and
discharges from agriculture. Agriculture is the predominant land use in the catchment
(61 per cent) and some of the highest production intensive dairy and poultry farming in the
state of Virginia is located in the catchment.
Community consultation was undertaken to help determine which best management practices
(BMPs) should be implemented at the watershed scale to achieve the TMDL. Total nitrogen
losses were managed through BMPs even though the goal was to reduce surface water nitrate
levels. The development of farm plans, nutrient management plans, assistance with individual
BMP identification (based on a farm’s risk assessment), and funding assistance (where
applicable) is delivered on the ground by the local conservation district (in this case, the
Shenandoah Valley Soil and Water Conservation District).
Virginia state uses the national Leaching Index for assessing nitrogen risk, except for animal
concentration areas where stock is fed in a fixed location (such as feedlots). These practices
are assessed using the Risk Assessment for Water Quality Impairment from Animal
Concentration Areas. This is a more sophisticated index than the Leaching Index, because
it incorporates farm-specific management and environmental risk factors (beyond soil
type and climate).
BMP implementation began in the early 2000s. By 2003, the area of Muddy Creek used for
drinking water no longer breached the drinking water standard, and the trend continued. In
2010, the stretch of river was removed from the list of impaired waters for nitrate-nitrogen
impairment. The watershed remains impaired for aquatic life due to reduced dissolved
oxygen levels.
Riparian buffer and stock exclusion fencing on Muddy Creek.
Image courtesy of Shenandoah Valley Soil and Water Conservation District
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